1. Field of Invention
The present invention relates generally to methods and apparatus for milling windows in well casings in the downhole environment whenever the trajectory of a well should be modified after a casing or liner has been set in a well or when one or a plurality of branches are built from a parent well. More particularly, the present invention concerns a method and apparatus for milling casing windows which ensures predictable milling so that the resulting casing window will be of predetermined dimension, contour geometry, location and orientation. Even more specifically, the present invention provides for stabilized rotation and efficiently controlled guiding of a pilot mill having articulated and rotary driven relation with a substantially rigid string mill, especially during initiation of casing milling, to ensure efficient deflector controlled guiding of the pilot mill and guiding of the string mills by the pilot mill, to ensure precisely controlled formation of a casing window by the pilot mill and string mills. The present invention also concerns a casing window milling system incorporating an articulated pilot mill having the capability for controlling its amplitude of relative misalignment with a substantially rigid milling shaft and having rotary driven relation with the milling shaft during initiation of casing milling and during initial pilot boring into the subsurface formation from the casing window.
2. Related Art
Casing windows are required whenever the trajectory of a well should be modified after a casing or a liner has been set in a well or when one or a plurality of branches are built from a parent well.
A casing window is generally performed with a combination of mills mounted on a mandrel at the bottom end of a drill string and wedging between the casing and a deflection tool called the whipstock. The whipstock is generally set in the hole in combination with the first milling run. The window may be completed in one single operation in the hole or in multiple runs. The peripheral surface of mills is generally covered with abrasive or cutting inserts made of hard material such as sintered tungsten carbide compounds brased on a steel mandrel. The hardness of the whipstock is generally designed so minimum wear will be generated by the rotation of mills peripheral surface onto the whipstock face while the assembly is pushed and rotated against the casing wall under deflecting action of the whipstock. However the milling action generally results from unbalanced pressures between respectively the mill(s) and the whipstock on one hand and the mill(s) and the casing wall on the other hand.
In high inclination condition, the whipstock face is generally oriented upward and therefore forces applied by the mill(s) onto the whipstock face increase with the increasing weight component of the milling string. Although a whipstock is expected to support some milling damage, how much whipstock material is left after milling has been preformed is difficult to predict. In such case the success of whipstock retrieval may become risky and lead to lost time and additional contingency and sometimes to the loss of the bottom section of the well.
The lack of control on the window geometry is another major disadvantage of conventional window milling techniques and makes some lateral branching techniques inapplicable or more complex. Most windows show a lower section directed sideways with respect to the hole axis. How much this "walk away" affects a window is hardly predicable and depends on several factors like well inclination, pilot mill size and shape, mill cutting structure, weight on bottom hole assembly, whipstock hardness and orientation.
When the formation surrounding the well casing being penetrated by the window bore is well consolidated, it is desirable that the pilot mill have a geometry enabling it to be efficiently guided along an intended trajectory by the wall surface of the wellbore being formed. When the formation surrounding the wellbore is not well consolidated, a pilot mill which has a freely articulated and rotary driven connection with a substantially rigid milling shaft could be subject to forces that might tend to change its course from the intended trajectory. If the pilot mill should be suddenly articulated when encountering some unusual structure in the downhole environment, the pilot mill or its articulated connection with the milling shaft could become damaged, perhaps to the extent of being separated from the milling shaft. It is desirable therefore to provide a casing window milling system having an articulated pilot mill and also having a mechanism for controlling the amplitude of relative misalignment of the pilot mill relative to the axis of rotation of the milling shaft. This pilot mill amplitude control feature will permit the pilot mill to be efficiently deflected so as to follow the slope of the deflecting tool without damaging the deflecting tool and will permit the pilot mill to be constrained in a coaxial relationship with the milling shaft so as to be guided by the milling shaft after the pilot mill has passed a point on the deflecting tool where self guiding of the pilot mill can no longer be ensured. Thus it is desirable to provide a casing window milling tool which incorporates a locking or restraining mechanism which can be actuated mechanically or hydraulically to lock the pilot mill in co-axial, stabilized relation with the milling shaft.